Electron beam apparatus typically includes a hollow housing and a "gun" or source of energetic electrons mounted within the housing. The interior or the housing is maintained under vacuum so as to facilitate generation and direction of the electron beam. In many types of electron beam devices, the electron beam does not pass out of the housing. For example, in a common cathode ray tube, the beam acts only on a phosphor inside the housing to produce visible light, which in turn is transmitted through a transparent wall of the housing. Likewise, in certain electron beam material treatment apparatus, such as electron beam welding devices and the like, the workpiece to be treated by the electron beam is placed into the housing and the housing is then evacuated before operation.
Other types of electron beam apparatus require that the beam pass out of the tube housing. The electron tube housing is provided with an aperture or "window" for passage of the electron beam, so that the beam can be directed on a workpiece positioned outside of the housing. For example, in electron beam sterilization and chemical curing processes, an item to be treated is positioned outside of the housing in front of the window and treated by the beam. In electron beam printing processes, a document to be printed is positioned outside of the housing, in front of the window, and treated with an electron beam so as to apply an electrical charge on the document in a manner corresponding to the pattern of the desired printing. My own U.S. Pat. No. 5,093,603 and PCT International Publication No. W0/91/07772 disclose methods and devices in which an electron beam is used to promote dispersion of a fluent material such as a liquid, slurry or gas-borne powder. In this arrangement, the electron beam is generated inside an evacuated housing and passes out of the housing through an opening or window so that the beam impinges upon the fluent material.
The opening or window in the electron tube housing must be covered with a membrane which permits passage of the electron beam to the outside of the housing, but which blocks passage of air or other fluids into the housing so as to preserve the vacuum within the housing. For example, in fluid dispension methods according to the aforementioned patent and application, the fluid typically is at atmospheric or superatmospheric pressure. Thus, the membrane must allow passage of the electron beam out of the tube and into the fluid while isolating the interior of the housing from the fluid.
Membranes utilized for passage of an electron beam must meet numerous conflicting requirements. The membrane must have relatively low electron absorption so that the beam passes through the membrane with little attenuation. This is significant both with respect to the power remaining in the beam and with respect to possible effects of the electron beam on the membrane itself. Thus, where the membrane absorbs a substantial fraction of the electrons in the beam, the energy imparted by the electrons may heat the membrane to an unacceptable degree or otherwise destroy the membrane. The requirement for low absorption leads to a strong preference for very thin membranes formed from materials having inherently low electron absorptivity, typically materials formed from elements having low atomic number. The membrane must provide an effective barrier against entry of atmospheric or other materials into the interior of the tube housing. It should be substantially impermeable to common gases and liquids, and must have sufficient physical strength to resist differential pressure encountered in service. As the interior of the housing is maintained substantially under vacuum, the differential pressure applied to the membrane is substantially equal to the absolute pressure prevailing on the outside of the housing in the vicinity of the membrane. Where the exterior surface of the membrane is exposed to a fluent material under superatmospheric pressure, high differential pressures are encountered. The differential pressure causes considerable stress in the membrane. Moreover, the fluid pressure may fluctuate, and hence the stress applied to the membrane may be a fluctuating stress. These factors require that the membrane have considerable mechanical strength.
Further, the tube housing may be subjected to substantial temperature changes during manufacture and service. It is normally necessary to subject an electron tube to a so-called "bakeout" treatment at elevated temperature during manufacture. Typically, the bakeout procedure is conducted after the electronic components of the tube such as electrodes, coils and the like have been mounted inside of the housing but before the housing has been fully sealed. The elevated temperature drives off volatile materials from the inside of the housing and from the electronic components. The heating and cooling which occurs during the bakeout process can induce significant thermal expansion and contraction of the membrane and housing, leading to still further stresses. The magnitude of such stresses is directly proportional to the difference between the coefficients of thermal expansion of the membrane material and the coefficient of thermal expansion of the adjacent housing material.
All of these factors taken together present a significant technical challenge. Moreover, in many applications the cost of the electron tube structure is of significance.
Considerable effort has been devoted in the art to the search for an electron tube structure and methods of making electron tubes which satisfy the foregoing considerations. Neukermans, U.S. Pat. No. 4,468,282 discloses an electron beam window structure and methods of making the same in which a window material such as boron carbide (B.sub.4 C) or other similar material is deposited on a substrate by chemical vapor deposition. The substrate is then etched to form a hole in alignment with the deposited window material. The substrate forms a wall of the electron tube housing, and the etched hole constitutes the window opening. VanRalte et al, U.S. Pat. No. 3,788,892 forms an opening in the wall of the housing and covers that opening with a temporary support film. The window material is then deposited in a relatively thin layer over temporary support film. The deposited window material extends beyond the periphery of the temporary support film, so that the deposited window material bonds with the housing wall. After deposition of the window material, the temporary support film is removed, dissolving the same. Another reference directed to fabrication of electron beam permeable membranes is Japanese Laid-Open Patent Publication 2-138900. U.S. Pat. Nos. 3,531,340; 5,030,318; and 4,228,815 describe fabrication of thin, membrane-like structures for other purposes.
Various attempts have been made to select structural configurations for the windows opening, of the housing and associated components so as to maximize the pressure resistance of the window. Much of this work has been directed to optimization of large area electron beam window structures, having a window area (measured in the plane of the membrane) on the order of 1 cm.sup.2 or more, and typically 100 cm.sup.2 or more. These large-window structures typically incorporate a supporting framework with multiple apertures and a unitary membrane extending across the various apertures. Structures of this type are described, for example, in U.S. Pat. Nos. 4,721,967; 4,333,036 and 4,591,756. A further electron beam window structure is shown in U.S. Pat. No. 3,105,916.
Despite all of this effort in the art heretofore, there have been substantial, unmet needs heretofore for improved electron tube structures equipped with electron permeable membranes; for improved methods of making such structures; and for improved components for use in fabricating such structures.
The present invention addresses these needs.